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Easter eggs may be good for you, but only if you eat small ones made from cocoa-rich dark chocolate, according to a German study.

The researchers from the German Institute of Human Nutrition found people who ate the most chocolate - an average of 7.5 grams a day - had lower blood pressure and a 39% lower risk of having a heart attack or stroke than those who ate the least amount of chocolate - an average of 1.7 grams a day.

But, the difference between the two groups was just under six grams of chocolate a day, less than one small square of an average 100 gram bar.

Lead researcher Dr Brian Buijsse, says people should not use his work as an excuse to stuff themselves with chocolate.

"Small amounts of chocolate may help to prevent heart disease, but only if it replaces other energy-dense food, such as snacks, in order to keep body weight stable," he says.

Although they said more work needed to be done to be sure, the researchers think the flavanols in cocoa may be the reason why chocolate seems to be good for blood pressure and heart health. Since there is more cocoa in dark chocolate, dark chocolate may have a greater effect.

Relaxes blood vessels

Flavanols are a class of the antioxidant flavonoids that are found in many vegetables, cocoa and red wine.

"Flavanols appear to be ... responsible for improving the bioavailability of nitric oxide from the cells that line the inner wall of blood vessels," says Buijsse.

Nitric oxide is a gas that, once released, causes the smooth muscle cells of the blood vessels to relax and widen, he says, adding that this may contribute to lower blood pressure.

For their chocolate study, the researchers used data from participants of a larger study called European Prospective Investigation into Cancer (EPIC).

They followed more than 19,300 over more than 10 years during which time their blood pressure, height and weight measurements as well as details of their diet, lifestyle and health were recorded.

Buijsse says in terms of absolute risk, the findings showed that if people in the group eating the least amount of chocolate increased their chocolate intake by six grams a day, 85 fewer heart attacks and strokes per 10,000 people could be expected to occur over a period of about 10 years.

Packed with calories

Commenting on the study on behalf of the European Society of Cardiology, Assistant Professor Frank Ruschitzka of Switzerland's University Hospital Zurich says basic science has now demonstrated "quite convincingly" that dark chocolate with a cocoa content of at least 70% reduces some kinds of stress and can improve blood flow and blood pressure.

But he says, "Before you rush to add dark chocolate to your diet, be aware that 100 grams ... contains roughly 500 calories.

"You may want to subtract an equivalent amount of calories by cutting back on other foods to avoid weight gain."